U.S. patent number 5,378,142 [Application Number 07/684,409] was granted by the patent office on 1995-01-03 for combustion process using catalysts containing binary oxides.
This patent grant is currently assigned to Engelhard Corporation. Invention is credited to Ting C. Chou, Robert J. Farrauto, John K. Hochmuth, Teresa Kennelly.
United States Patent |
5,378,142 |
Kennelly , et al. |
* January 3, 1995 |
Combustion process using catalysts containing binary oxides
Abstract
A catalyst composition containing one or more binary oxides of
palladium and rare earth metal such as Ce, La, Nd, Pr and/or Sm.
The catalyst composition is used for the catalytic combustion of
gaseous combustion mixtures of oxygen and carbonaceous fuels such
as methane, e.g., a natural gas/air combustion mixture. Specific
preferred binary oxides may be, for example, M.sub.2 O.sub.3.sup..
PdO (e.g., La.sub.2 O.sub.3.sup.. PdO) or 2M.sub.2 O.sub.3.sup..
PdO, wherein in each case M Is La, Nd or Sm. A process of
combusting gaseous carbonaceous fuels includes contacting a
catalyst as described above under combustion conditions, e.g.,
925.degree. C. to 1650.degree. C. and 1 to 20 atmospheres pressure,
to carry out sustained combustion of the combustion mixture,
including catalytically supported thermal combustion. Regeneration
of over-temperatured M.sub.2 O.sub.3.sup.. PdO catalyst is also
provided for.
Inventors: |
Kennelly; Teresa (Belle Mead,
NJ), Hochmuth; John K. (Monmouth Junction, NJ), Chou;
Ting C. (San Jose, CA), Farrauto; Robert J. (Westfield,
NJ) |
Assignee: |
Engelhard Corporation (Iselin,
NJ)
|
[*] Notice: |
The portion of the term of this patent
subsequent to December 8, 2009 has been disclaimed. |
Family
ID: |
24747926 |
Appl.
No.: |
07/684,409 |
Filed: |
April 12, 1991 |
Current U.S.
Class: |
431/7; 502/303;
502/304; 502/339; 60/723; 502/262; 502/302; 502/263; 502/38;
423/213.5; 60/772; 502/355 |
Current CPC
Class: |
B01J
23/63 (20130101); B01J 38/12 (20130101); B01J
23/96 (20130101) |
Current International
Class: |
B01J
23/63 (20060101); B01J 23/54 (20060101); B01J
23/90 (20060101); B01J 23/96 (20060101); F23D
003/40 (); B01J 038/12 (); B01J 023/10 (); B01J
023/56 () |
Field of
Search: |
;431/7,170 ;423/213.5
;502/38 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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244339 |
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Dec 1985 |
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JP |
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140646 |
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Jun 1987 |
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JP |
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216642 |
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Sep 1987 |
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JP |
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088041 |
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Apr 1988 |
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JP |
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238311 |
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Oct 1988 |
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JP |
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294411 |
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Dec 1988 |
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JP |
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296842 |
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Dec 1988 |
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JP |
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57002 |
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Mar 1989 |
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JP |
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169003 |
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Jun 1990 |
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JP |
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237643 |
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Sep 1990 |
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JP |
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2114016 |
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Jan 1983 |
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GB |
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Other References
Attfield et al "Structural Correlations . . . " J. Solid State
Chem. 80, 286-298 (1989). .
Attfield, "Ab Initio Structure Determinations of the La Pd Oxides .
. . " Acta Cryst, (1988)B44,563-568. .
Russian J. Inorg. Chem. 27(8), 1982 p. 1180. .
Kato et al, "Lanthanide B-Alumina Supports . . . " in Successful
Design of Catalysts, Inui (ed) (1988). .
McDaniel et al "Phase relations Between PdO . . . " J. of Research
of the Nat. B of Standards 72A vol. Jan.-Feb. 1968. .
Muller et al "Synthesis and Crystal Chemistry . . . " in Platinum
Group Metals and Compounds, ACS 1970. .
Cullis et al Trans. Far Soc. 8, 1406 (1972)..
|
Primary Examiner: Konopka; Paul E.
Claims
What is claimed is:
1. A process for the catalytically supported combustion of a
gaseous carbonaceous fuel which comprises (a) forming a gaseous
combustion mixture comprising the fuel and oxygen, and (b)
contacting the combustion mixture in a catalyst zone with a
catalyst composition comprising a refractory carrier on which is
disposed a catalytic material comprising a mixture of (i) a
refractory inorganic binder and (ii) a catalytically effective
amount of a powdered binary oxide of palladium and a rare earth
metal selected from the group consisting of one or more of Ce, La,
Nd and Sm, the contacting being carried out under conditions
suitable for catalyzed combustion of the combustion mixture,
thereby effecting sustained combustion of at least a portion of the
fuel in said combustion mixture without substantial formation of
oxides of nitrogen.
2. The process of claim 1 including conducting the catalyzed
combustion under substantially adiabatic conditions at a reaction
rate exceeding the mass transfer rate of gaseous fuel and oxygen to
the catalyst to attain catalytically supported thermal combustion
of at least a portion of the fuel in said combustion mixture.
3. The process of claim 1 wherein the binary oxide comprises an
oxide of the formula M.sub.2 O.sub.3.sup.. PdO, wherein M is the
rare earth metal and is selected from the group consisting of La,
Nd and Sm.
4. The process of claim 1 wherein the binary oxide comprises
La.sub.2 O.sub.3.sup.. PdO.
5. The process of claim 1 wherein the binary oxide comprises an
oxide of the formula 2M.sub.2 O.sub.3.sup.. PdO, wherein M is the
rare earth metal and is selected from the group consisting of La,
Nd and Sm.
6. The process of claim 1 including maintaining a temperature of
from about 925.degree. C. to 1650.degree. C. and a pressure of from
about 1 to 20 atmospheres in the catalyst zone.
7. The process of claim 6 including maintaining a temperature of
from about 1000.degree. C. to 1500.degree. C. and a pressure of
from about 1 to 15 atmospheres in the catalyst zone.
8. The process of claim 1 wherein the fuel comprises methane and
the oxygen is supplied by air.
9. The process of claim 8 wherein the combustion mixture comprises
about 95 to 99 volume percent air.
10. The process of claim 1 wherein the binary oxide has the formula
M.sub.2 O.sub.3.PdO, wherein M is the rare earth metal, and
including regenerating the catalyst composition after deactivation
thereof caused by heating the catalyst composition to a temperature
above its decomposition temperature, by heating the catalyst
composition in the presence of an oxygen-containing gas at a
regeneration temperature of about 790.degree. C. or less.
11. The process of claim 10 wherein the regeneration temperature is
from about 700.degree. C. to 790.degree. C.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to catalyst compositions comprising
oxides of rare earth metals and palladium which are suitable for
the combustion of gaseous carbonaceous fuels such as natural gas or
methane, and to a process for catalytic combustion of such fuels
using the catalyst compositions.
2. Related Art
Complexes of palladium oxide and rare earth metal sesquioxides are
known, as shown by an article by C. L. McDaniel et al, "Phase
Relations Between Palladium Oxide and the Rare Earth Sesquioxides
in Air", Journal of Research of the Natural Bureau of Standards -
A. Physics and Chemistry, Vol. 72A, No. 1, January-February, 1968.
Pages 27-37 describe complexes of PdO and rare earth metal oxides.
Specifically, the paper describes the study of equilibrium phase
relations in an air environment between PdO and each of the
following sesquloxides: Nd.sub.2 O.sub.3, Sm.sub.2 O.sub.3,
La.sub.2 O.sub.3, Eu.sub.2 O.sub.3, Gd.sub.2 O.sub.3, DY.sub.2
O.sub.3, Ho.sub.2 O.sub.3, Y.sub.2 O.sub.3, Er.sub.2 O.sub.3,
Tm.sub.2 O.sub.3, Yb.sub.2 O.sub.3 and Lu.sub.2 O.sub.3. The
experimental procedure employed is described In section 3 at page
28 and describes mixing various combinations of PdO and the rare
earth metal oxides and subjecting the mixture to preliminary heat
treatments for a minimum of 18 hours at 770.degree. C. and then at
780.degree. C. After each heat treatment the materials were
examined by x-ray diffraction techniques and following the
preliminary heat treatments portions of each batch were fired at
various temperatures, typically lying between 1000.degree. C. and
3000.degree. C. as set forth in TABLE I, pages 29-33. Among other
findings, the paper notes (in the summary of section 4.3 at page
34) the dissociation temperature of PdO in air at atmospheric
pressure to be 800.degree. C..+-.5.degree. C., and that palladium
oxide reacts with a number of the rare earth metal oxides to form
binary compounds. What is described as the pseudobinary system
Nd.sub.2 O.sub.3.sup.. PdO is said to exemplify the typical type of
reaction and three binary oxide compounds of, respectively, 2:1,
1:1 and 1:2 ratios of Nd.sub.2 O.sub.3 :PdO are disclosed, viz,
2Nd.sub.2 O.sub.3.sup.. PdO; Nd.sub.2 O.sub.3.sup.. PdO and
Nd.sub.2 O.sub.3.sup.. PdO. (The compound 2Nd.sub.2 O.sub.3.sup..
PdO may of course be written as Nd.sub.4 PdO.sub.7.) Analog
compounds are noted for the Sm.sub.2 O.sub.3.sup.. PdO, Eu.sub.2
O.sub.3.sup.. PdO and La.sub.2 O.sub.3.sup.. PdO systems, with only
the 2:1 and 1:2 compounds occurring in the latter system. However,
it was noted that other rare earth oxide-palladium oxide
combinations did not react in the solid state. There were
combinations of PdO with, respectively, Ho.sub.2 O.sub.3, Y.sub.2
O.sub.3, Er.sub.2 O.sub.3, Tm.sub.2 O.sub.3, Yb.sub.2 O.sub.3 and
Lu.sub.2 O.sub.3. (See the Abstract at page 27, and the last
sentence on page 35.)
Another article, by A. Karo et al, "Lanthanide B-Alumina Supports
For Catalytic Combustion Above 1000.degree. C.", Successful Design
of Catalysts, 1988 Elsevier Science Publishers, pages 27-32,
describes the preparation of support materials consisting of
lanthanide oxides and alumina for use as combustion catalysts. The
article states that endurance tests on methane combustion performed
at 1200.degree. C. proved that a Pd catalyst supported on lanthanum
beta-alumina has good resistance to thermal sintering (page
32).
An English language abstract of Japanese Patent Publication J
63088041 (1988) TsuJ et al, Tanaka Kikinzoku Kogyo discloses a
catalyst consisting of a ceramic support composed of alumina on
whose surface Pd and/or PdO is supported on a layer of
intermetallic PdM or PdMO wherein M is a rare earth metal. The
disclosed use is for catalyzing combustion of fuel without
generation of nitrogen oxides. Chemical Abstracts (CA 109 11 32
93k) identifies the patentee as Tanaka Noble Metal-Industrial Co.
Ltd. and describes the methane combustion catalyst as
Pd-Pd/LaO.sub.2 on gamma-alumina and reports that this catalyst
cornbusted natural gas at an inlet temperature of 355.degree. C.
with 100% efficiency after 1 hour and with 99.98% efficiency after
1,000 hours.
U.S. Pat. No. 4,893,465 issued to Robert J. Farrauto et al
describes a process for the catalytic combustion of carbonaceous
materials, such as natural gas or methane, using a palladium oxide
containing catalyst. In the process, the palladium oxide catalyst
for the catalytic combustion Is subjected to temperatures in excess
of the decomposition temperature of palladium oxide to metallic
palliadium, the latter being inactive for catalysis of the
combustion reaction. At atmospheric pressure the decomposition
temperature of PdO is at least about 800.degree. C. The stated
improvement in the process of the Patent comprises restoring
catalytic activity by lowering the temperature of the catalyst to a
regenerating temperature, i.e., a temperature at which Pd is
oxidized to PdO, which in air at atmospheric pressure is from about
530.degree. C. to about 650.degree. C., and maintaining the
temperature within that range until desired catalytic activity is
achieved by re-oxidation of catalytically inactive Pd to PdO. The
examples of the Patent utilize PdO/Al.sub.2 O.sub.3 as the
catalyst.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a
catalyst composition comprising a refractory carrier on which is
dispersed a catalytic material comprising a mixture of a refractory
inorganic binder (e.g., one or more of silica, alumina, titania and
zirconia) and a binary oxide of palladium and a rare earth metal.
The rare earth metal may be one or more of Ce, La, Nd, Pr and Sm.
For example, in one aspect of the invention the binary oxide has
the formula 2M.sub.2 O.sub.3.sup.. PdO, where M is one or more of
La, Nd and Sm; in another aspect of the invention, the binary oxide
has the formula M.sub.2 O.sub.3.sup.. PdO, where M Is one or more
of La, Nd and Sm, e.g., La.sub.2 O.sub.3.sup.. PdO.
In another aspect of the invention the binary oxide is contained In
a reaction product obtained by heating a reaction mixture of
palladium oxide and a rare earth metal oxide to an elevated
temperature for a time sufficient to react at least a portion of
the palladium and rare earth metal oxides to form the binary oxide.
In one embodiment of this aspect of the invention, the reaction
mixture comprises a mixture in the proportion of about two moles of
M.sub.2 O.sub.3 to one mole of PdO and the reaction product
contains the binary oxide having the formula 2M.sub.2 O.sub.3.sup..
PdO, where M Is one or more of La, Nd and Sm. Another embodiment of
this aspect of the invention provides as the reaction mixture a
mixture in the proportion of about one mole of La.sub.2 O.sub.3 to
one mole of PdO and the reaction product contains La.sub.2
O.sub.3.sup.. PdO as the reaction product.
The invention provides in certain aspects thereof that the carrier
comprises a body having a plurality of parallel gas flow passages
extending therethrough, the passages being defined by walls on
which the catalytic material is disposed as a coating, i.e., as a
"washcoat".
A process aspect of the invention provides a process for the
catalytically supported combustion of a gaseous carbonaceous fuel.
The process comprises the steps of forming a gaseous combustion
mixture comprising the fuel and oxygen, for example, a mixture of
natural gas or methane and air, and contacting the combustion
mixture in a catalyst zone with a catalyst composition. The
catalyst composition comprises a refractory carrier on which is
disposed a catalytic material comprising a mixture of a refractory
inorganic binder (e.g., one or more of silica, alumina, titania and
zirconia) and a binary oxide of palladium and a rare earth metal
selected from the group consisting of one or more of Ce, La, Nd, Pr
and Sm. The contacting is carried out under conditions in the
catalyst zone, e.g., a temperature of from about 925.degree. C. to
1650.degree. C. and a pressure of about 1 to 20 atmospheres, which
are suitable for catalyzed combustion of the combustion mixture, so
that sustained combustion of at least a portion of the fuel in the
mixture is thereby effected. A narrower range of suitable
combustion conditions may be maintained in the catalyst zone, for
example, a temperature of about 1000.degree. C. to 1500.degree. C.
and a pressure of about 1 to 15 atmospheres.
In another process aspect of the invention, the catalyzed
combustion is carried out under substantially adiabatic conditions
at a reaction rate exceeding the mass transfer rate of gaseous fuel
and oxygen to the catalyst, to attain catalytically supported
thermal combustion of at least a portion of the fuel without any
substantial formation of oxides of nitrogen.
In another aspect of the present invention, there is provided a
process for regenerating a catalyst composition comprising a
refractory carrier on which is disposed a catalytic material
comprising a mixture of (i) a refractory inorganic binder and (ii)
a catalytically effective amount of a binary oxide having the
formula M.sub.2 O.sub.3.sup.. PdO wherein M is selected from the
group consisting of one or more of La, Nd and Sm (e.g., the binary
oxide may be La.sub.2 O.sub.3.sup.. PdO) and which has sustained
deactivation caused by being heated to a temperature above its
deactivation temperature. The regeneration comprises heating the
catalyst composition in the presence of an oxygen-containing gas at
a regeneration temperature of about 790.degree. C. or less, e.g.,
in the range of from about 700.degree. C. to 790.degree. C. The
oxygen-containing gas may be air.
Other aspects of the invention provide using as the catalyst
composition the compositions described above, and still others
aspects of the invention are described below.
BRIEF DESCRIPTION OF THE DRAWING
The sole Figure is a plot of a thermogravimetric analysis ("TGA")
in air of a sample of a reaction product containing the binary
oxide La.sub.2 O.sub.3.sup.. PdO of the invention, showing
temperature versus sample weight and illustrating weight changes
associated with decomposition of the compound to catalytically
inactive species and regeneration of the catalytically inactive
species to catalytically active La.sub.2 O.sub.3.sup.. PdO.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
THEREOF
There is interest in using gaseous carbonaceous fuels such as
natural gas or other carbonaceous fuels for various applications,
including fueling gas turbines. Natural gas is a mixture of low
molecular weight paraffin series hydrocarbons such as methane,
ethane, propane and butane with small amounts of higher
hydrocarbons, with methane almost always being the major
constituent. Other suitable fuels include ethane, propane, butane,
other hydrocarbons, alcohols, other carbonaceous materials, and
mixtures thereof. Thermal combustion of gaseous carbonaceous fuels
such as natural gas and/or other carbonaceous fuels takes place at
high temperatures, In excess of 1650.degree. C., where NOx
formation occurs by oxidation of atmospheric nitrogen in the
combustion air. The use of a catalyzed combustion process which
operates at lower temperatures than a flame combustion process, for
example, within a temperature range of about 925.degree. C. to
1650.degree. C., would significantly reduce or eliminate the
formation of nitrogen oxides. However, the most prevalent gaseous
carbonaceous fuel is natural gas. Combusting methane, which is the
predominant component of natural gas, is difficult because one of
the steps in the oxidation of methane is the cleavage of a C--H
bond which is more difficult to accomplish in methane than cleavage
of either C--H or C--C bonds of higher carbon number hydrocarbons.
Therefore, catalysts which are active for oxidation of higher
carbon number hydrocarbons may not be active, or sufficiently
active, for oxidation of methane or methane-containing gases.
Further, the catalysts employed for catalytic combustion of
carbonaceous gases must also remain active at the temperatures to
which they are exposed during catalyzed combustion. Catalytic
combustion temperatures may range, as noted above, from about
925.degree. C. to 1650.degree. C. More usually, the range is about
1000.degree. C. to 1500.degree. C., but the temperature to which a
given segment of catalyst is exposed must be controlled to below
the decomposition temperature of the catalyst. For example,
palladium oxide (PdO) shows excellent activity for such catalytic
combustion but, at atmospheric pressure, is deactivated at a
temperature of about 800.degree. C. or so. References herein and in
the claims to decomposition and regeneration temperatures of the
various binary oxide catalytic species are all at atmospheric
pressure, it being understood that at enhanced partial pressure of
oxygen the decomposition and regenerating temperatures will shift
upwardly. The determination of such increased temperatures at
higher oxygen partial pressures will be a matter well known to
those skilled in the art. Thus, the above stated temperature ranges
are dependent on the partial pressure of oxygen, and at higher
pressures, as for example might be encountered in conjunction with
generation of combustion effluent useful for operation of gas
turbines, the decomposition temperatures of the binary oxide
species will increase, as will the regeneration temperature at
which they will re-form.
The present invention provides a catalyst composition comprising
one or more binary oxides of palladium and rare earth metal: such
catalyst compositions have been found to be highly heat resistant
as compared to PdO. Although such binary oxides are less active
than PdO, their ability to operate at temperatures much higher than
the 800.degree. C. deactivation temperature of PdO is advantageous.
Further, if such binary oxide catalysts are accidentally exposed to
temperatures high enough to inactivate them, some of them can be
regenerated, in some cases, at relatively high temperatures. These
characteristics make the binary oxides used in the present
invention extraordinarily useful as catalysts four catalytic
combustion of carbonaceous gases, especially methane and
methane-rich gases such as natural gas.
The binary oxide catalysts of the present invention may be prepared
by either a dry mixed-oxide method or a solution-drying method. In
the dry mixed-oxide method an oxide of a rare earth metal, for
example, one selected from the group consisting of one or more of
Ce, La, Nd, Pr and Sm, is mixed with palladium oxide in selected
weight ratios. The mixture is mechanically ground to a size range
of about 50 to 100 micron diameter particles. The grinding is
followed by calcination in air, for example, at a temperature of
about 1100.degree. C. for about 66 hours, to provide a reaction
mixture containing the binary oxide of palladium and rare earth
metal of the invention. Preferably, the rare earth metal oxide and
palladium oxide starting materials are mixed in stoichiometric
proportions to produce the desired compound. Thus, the molar ratio
of the rare earth metal oxide (e.g., M.sub.2 O.sub.3, where M is
Ce, La, Nd, Pr or Sm) to PdO in the reaction mixture may be 2:1,
1:1 or 1:2 molar ratios of M.sub.2 O.sub.3 to PdO. Although it is
not necessary to use the starting materials in the molar ratios of
the desired binary oxide product, the use of such stoichiometric
proportions has been found to be advantageous particularly with
respect to the preparation of the 2M.sub.2 O.sub.3.sup.. PdO
compounds as discussed in more detail below.
In the solution-drying method, suitable amounts (preferably the
same 2:1, 1:1 or 1:2 molar ratios) of a nitrate or other suitable
soluble compound of the rare earth metal and a suitable, soluble
palladium salt such as palladium nitrate,
Pd(NO.sub.3).sub.2.2H.sub.2 O, are mixed in an aqueous solution at,
for example, temperatures of between about 60.degree. C. and
90.degree. C., and heated to evaporate to dryness. The resulting
residue is calcined in air, for example, at temperatures from
500.degree. C. to about 1100.degree. C. for about 2 to 18 hours to
provide a reaction mixture containing the binary oxide of the
invention.
The use of x-ray diffraction analysis showed the presence in the
reaction products of both the dry mixed-oxide and solution-drying
methods of the desired binary oxide of palladium and the rare earth
metal.
The physical configuration of a catalyst composition of this
invention may utilize as a carrier a structure comprising a ceramic
substrate on which is disposed a coating of a catalytic material,
conventionally referred to as a "washcoat". In the case of the
present invention the washcoat is comprised of the binary oxide
catalyst and a suitable refractory binder. The ceramic substrate
body is usually cylindrical in shape and has a plurality of fine
gas flow passages extending therethrough, from an inlet to an
outlet face of the body, to provide a somewhat honeycomb-type
structure. The gas flow passages (sometimes referred to as "cells")
in the honeycomb structure are substantially parallel and defined
by thin walls, and may be of any desired cross section such as
square, other rectangular, triangular or hexagonal shape. The
number of channels per square inch of face surface may vary,
depending upon the particular application for which the catalyst
composition is to be used. Such honeycomb-type carriers are
commercially available having anywhere from about 9 to 600 cells
per square inch. The substrate or carrier (which is loosely
referred to as "ceramic" and may be made of cordierite, mullire,
silica, alumina or any such suitable material) desirably is porous
and may (but need not) be relatively catalytically inert to the
combustion reaction as compared to the binary oxide(s) used in the
invention. The catalyst compositions of the present invention may
be prepared according to known preparation techniques, viz, the
catalytic material containing the binder and binary oxides is
applied to the carrier by dipping the carrier into an aqueous
slurry of fine particles of the catalytic material, in order to
coat the gas flow passage walls. Excess slurry is removed, e.g., by
blowing it out of the gas flow passages with compressed air, and
the coated structure is dried and then calcined in air at
temperatures of about 500.degree. C. for about 2 hours to provide
an adherent "washcoat" of catalytic material on the walls defining
the gas flow passages.
Two classes of rare earth metal-palladium binary oxides which have
been found to be particularly useful in the practices of the
invention are those having the formulas, respectively, M.sub.2
O.sub.3.sup.. PdO and 2M.sub.2 O.sub.3.sup.. PdO, wherein in both
cases M is selected from the group consisting of one or more of La,
Nd and Sm. These catalysts display good activity for catalyzing the
combustion of carbonaceous gases, including methane, the most
difficult to combust of the carbonaceous gases normally utilized as
combustion fuels. The binary oxides M.sub.2 O.sub.3.sup.. PdO,
e.g., La.sub.2 O.sub.3.sup.. PdO, initially decompose to
catalytically inactive species at about 955.degree. C. but after
being aged for several heating and cooling cycles, display a
decomposition onset temperature and a regeneration onset
temperature of about 790.degree. C. The near co-incidence of the
decomposition and regeneration onset temperatures substantially
eliminates the hysteresis or "dead zone" gap between catalytically
active M.sub.2 O.sub.3.sup.. PdO and catalytically inactive
species. This greatly facilitates regeneration of catalysts which
are accidentally inactivated by inadvertent over-temperature
operation. On the other hand, whereas the binary oxides 2M.sub.2
O.sub.3.sup.. PdO as a practical matter are not significantly
regenerable once they have decomposed, they do not decompose to
catalytically inactive species until temperatures well in excess of
1200.degree. C. are reached.
The features and advantages of certain embodiments of the present
invention are illustrated with respect to the following
non-limiting Examples.
EXAMPLE 1
A. A stoichiometric mixture of two moles of lanthanum oxide
(La.sub.2 O.sub.3) and one mole of palladium oxide (PdO) powder was
ground to a fine particle size, calcined in air at 780.degree. C.
for 17 hours, reground, and calcined in air for an additional 66
hours at 1100.degree. C., then ground again to a final particle
size of about 100 to 150 microns in diameter to provide a powder of
the binary oxide 2La.sub.2 O.sub.3.sup.. PdO. A binder comprising
alumina sold under the trade name Catapal by Vista Company was
prepared by calcining the alumina in air at 950.degree. C. for 2
hours to reduce its surface area, thereby pre-stabilizing it
against thermal deactivation, and then grinding in a ball mill a
slurry of the calcined alumina at 45% solids in an aqueous medium
with 2.25 weight percent acetic acid. The grinding was continued to
a viscosity of 40 to 50 centipoise to provide a particle size of
the alumina in the range of less than about 20 microns diameter.
The resultant alumina binder and the 2La.sub.2 O.sub.3.sup.. PdO
powder were mixed to form an aqueous slurry in a proportion of 93
percent by weight alumina binder and 7 percent by weight 2La.sub.2
O.sub.3.sup.. PdO, dry basis. The finished washcoat slurry was
applied to a series of ceramic carrier bodies made of cordierite,
each being 1 inch in diameter, 1 inch long and having 400 cells per
square inch. This was accomplished by dipping each carrier into the
slurry, draining off excess slurry, and allowing the thus-coated
carrier to dry overnight at 120.degree. C. The carriers were then
calcined in air at 500.degree. C. for 2 hours to provide an
adherent washcoat of catalytic material on the carrier. Typically,
the resulting catalyst bodies contained 2.5 grams of washcoat per
cubic inch of volume, which provided 0.175 g of 2La.sub.2
O.sub.3.sup.. PdO per cubic inch of catalytic body.
B. Slurries of three more binary rare earth metal-palladium oxides
and one comparative material using PdO in place of the binary oxide
were prepared according to the procedure of part A of this Example
1. In all, three different binary oxides as set forth in TABLE I
below were prepared, as was a comparative catalyst which was coated
with the gamma alumina binder and 1% by weight (of the weight of
the washcoat) of PdO. The PdO was provided by Engelhard Corporation
and was 86% by weight Pd, as PdO. The catalyst bodies containing
the binary oxides of the invention each had an equivalent PdO
content of 1% by weight of the weight of the washcoat, the PdO
existing as part of the binary oxide compound in the case of the
catalysts of the invention.
C. The activity of all three of the catalysts was tested for
methane combustion by passing a gaseous stream of 1 volume percent
CH.sub.4 in air at ambient pressure through each of the catalyst
samples. A flow rate 20 liters per minute measured at standard
temperature and pressure was maintained, resulting in a space
velocity of 1,350,000 hours.sup.-1. During each test the
temperature of the inlet gas was gradually increased by externally
heating the bed and a Beckman Industrial Model 400A Hydrocarbon
Analyzer monitored the conversion of CH.sub.4 as a function of
temperature. As a measure of activity of the catalyst, the
temperature at which conversion of 20% of the inlet methane
occurred was taken. TABLE I lists the temperatures at which the 20%
conversion was reached for each catalyst composition.
TABLE I ______________________________________ Sample No. Catalyst
Temperature, .degree.C. ______________________________________ 1
2La.sub.2 O.sub.3.PdO 463 2 2Nd.sub.2 O.sub.3.PdO 512 3 2Sm.sub.2
O.sub.3.PdO 622 4 (Comparative) PdO 389
______________________________________
The data of TABLE I shows that the catalyst compositions comprising
the binary oxides of the present invention provide catalysts which
have acceptable activity, although clearly not as good as PdO.
Thus, the PdO-containing comparative catalyst of Sample 5 required
an inlet temperature of only 389.degree. C. to attain 20%
conversion of inlet CH.sub.4. However, the activity for catalyzed
combustion of Samples 1-3 in accordance with embodiments of the
present invention is well within acceptable operating temperature
ranges for catalytic combustion. Sample 1 (2La.sub.2 O.sub.3.sup..
PdO) shows the best activity of the catalysts exemplifying the
invention, with a temperature of 463.degree. C. being required to
attain 20% conversion of CH.sub.4, with the activities of Samples 2
(2Nd.sub.2 O.sub.3.sup.. PdO) and 3 (2Sm.sub.2 O.sub.3.sup.. PdO)
showing 20% conversion at 512.degree. C. and 622.degree. C.,
respectively. Although operation at temperatures of in excess of
about 800.degree. C. at atmospheric pressure will cause PdO to
deactivate into a catalytically inactive species, the binary oxide
compounds of the present invention are stable at much higher
temperatures, thereby permitting operation of the catalyst at well
above 800.degree. C. but still below the 1300.degree. C. or
1400.degree. C. temperatures at which significant nitrogen oxides
are generated. This is clearly advantageous as the higher
temperatures permit greater efficiencies of operation. Thus, the
binary oxides of the present invention are stable at temperatures
of up to about 1200.degree. C., enabling operation at about
400.degree. C. higher temperature than is attainable with PdO
catalysts. For example, a catalyst composition containing 2La.sub.2
O.sub.3.sup.. PdO is well suited for use in a staged catalytic
combustion device as a higher temperature stage catalyst, because
thermal decomposition of this catalyst occurs only at a temperature
above about 1200.degree. C.
EXAMPLE 2
The compounds 2Nd.sub.2 O.sub.3.sup.. PdO and 2La.sub.2
O.sub.3.sup.. PdO were prepared in accordance with the method of
Example 1. These materials were tested for stability in air by
thermogravimetric analyses carried out by measuring small weight
changes in a sample of the reaction product occurring at different
temperatures during a heating and cooling cycle. The weight changes
are caused by various chemical reactions and phase changes
undergone by the samples. A carefully weighed sample of 20
milligram ("mg") to 50 mg size is placed in a quartz pan which Is
suspended from a weight measuring device manufactured by Thermal
Sciences, Model STA 1500. Air at approximately 20 cm.sup.3 /min ds
passed over the sample. A furnace gradually heats the sample at a
rate of about 10.degree. C./min and weight changes occasioned by
the heating are noted to show the weight losses which result from
decomposition of the compound and consequent loss of oxygen. The
heating was continued until a temperature of 1400.degree. C. was
attained. In addition to the weight changes, temperature changes
caused by the heat of reaction (differential thermal analysis) of
the sample as decomposition takes place were monitored during the
heating period.
The results of the thermogravimetric analyses of the catalysts show
stability of the compounds to be as follows:
______________________________________ Compound Onset of
Decomposition .degree.C. ______________________________________
2Nd.sub.2 O.sub.3.PdO 1230.degree. C. 2La.sub.2 O.sub.3.PdO
1300.degree. C. ______________________________________
The thermogravimetric analyses of Example 2 indicated that the
tested 2M.sub.2 O.sub.3.sup.. PdO compounds are thermally stable in
air at atmospheric pressure up to the indicated temperatures, well
above 1200.degree. C. When heated to the indicated decomposition
temperatures, the catalytically active binary oxide compounds
2M.sub.2 O.sub.3.sup.. PdO are decomposed and the result is
formation of a catalytically inactive species. The thermal
stability and catalytic activity of the 2M.sub.2 O.sub.3.sup.. PdO
compounds for oxidation of carbonaceous gaseous fuels, makes these
compounds and reaction mixtures containing one or more of them well
suited for practical use in catalytic combustion processes for
carbonaceous gaseous fuels. For example, by way of comparison, PdO
decomposes to an inactive species at about 800.degree. C. more than
400.degree. C. below the decomposition temperatures of the 2M.sub.2
O.sub.3 .sup.. PdO binary oxides.
EXAMPLE 3
A. A catalytic combustion system was prepared as follows with two
sections of catalyst, a first or upstream section positioned to be
first contacted by the flowing gaseous combustion mixture stream,
and a second or downstream section being last contacted by the
flowing gaseous stream.
B. The first section was a catalyst body provided by a cordierite
honeycomb having 400 cells per square inch of end face and being 1
inch (2.54 cm) in diameter, and 4 inches (10.2 cm) long and coated
with alumina washcoat containing 8% PdO by weight of the dried,
calcined washcoat.
C. The second section was a catalyst body provided by a honeycomb
made of silica-alumina-magnesia fibers and of the same overall
dimensions as used for the first section, but with 64 cells per
square inch, coated with a washcoat of alumina mixed with 2La.sub.2
O.sub.3.sup.. PdO particles and having a content of 1% by weight
PdO equivalent contained in the binary oxide.
D. The combustion of methane was measured under the following
conditions. A gaseous stream containing about 4% by volume methane
In air was flowed at a velocity of 50 feet per second and at three
atmospheres pressure in sequence through the first and second
catalyst bodies. At an inlet temperature of 480.degree. C.,
combustion of methane was complete, unburned hydrocarbons and
oxides of nitrogen emissions were below 2 parts per million
("ppm").
The results of Example 3 show that a catalyst composition in
accordance with the invention may be utilized as a downstream
catalyst in combination with an upstream catalyst comprised of a
more active catalyst such as PdO. It is seen that the upstream or
inlet portion of the catalytic system operates at a lower
temperature than does the downstream portion. Consequently, a more
active catalyst but one which decomposes at relatively low
temperatures (such as a PdO catalyst) may be used in the cooler
upstream section whereas a catalyst, such as those provided by the
binary oxides of the present invention, which is more resistant to
high temperature may be used in the downstream or higher
temperature portions of the catalyst. Since the downstream section
is contacted by the higher temperature gases emerging from the
upstream section, the lower activity of the high temperature
resistant catalyst of the invention is nonetheless adequate to
catalyze combustion of the fuel at the higher, downstream
temperatures involved.
EXAMPLE 4
To ascertain the efficiency of the 2La.sub.2 O.sub.3.sup..
PdO-containing catalyst composition, a comparative test was made
with an arrangement similar to that used in Example 3, except that
the last 2 inches (as sensed in the direction of gas flow through
the catalyst bodies) of each catalyst body tested was provided in
one case with a washcoat containing the 2La.sub.2 O.sub.3.sup..
PdO/alumina mixture as in Example 2, and in the comparative case
with a washcoat of alumina without the binary oxide or any other
combustion catalyst. The data are given in the following TABLE
II.
At an inlet temperature of 475.degree. C., the total temperature
increment over the entire bed was 335.degree. C. and the
temperature increment over the last 2 inches was 200.degree. C. in
the case of 2La.sub.2 O.sub.3.sup.. PdO-containing washcoat. No
unburned hydrocarbons ("UHC") were detected in the effluent from
this catalyst body. The NOx content of the effluent was less than 1
ppm.
For the control or comparative catalyst body containing the alumina
washcoat without the 2La.sub.2 O.sub.3.sup.. PdO catalyst, the
inlet temperature was 480.degree. C. and the total temperature
increment was 200.degree. C., unburned hydrocarbons was 2.8% and
NOx was less than 1 ppm. The comparison shows that the 2La.sub.2
O.sub.3.sup.. PdO segment was an effective catalyst for combustion
of methane.
TABLE II ______________________________________ Comparative
Exemplary Sample Catalyst Sample
______________________________________ Gas Inlet Temp. (.degree.C.)
480 475 Fuel, Vol % CH.sub.4 4 4.1 Bed Temp. Increment,
(.degree.C.) 200 335 Bed Temp. Increment, N/A 200 last 2 inches
(.degree.C.) UHC in Effluent 2.8% ND NOx in Effluent <1 ppm
<1 ppm ______________________________________ UHC = unburned
hydrocarbons NOx = nitrogen oxides ND = not detected N/A = not
available
The efficacy of the binary oxide catalyst of the present invention
is shown by the much higher bed temperature attained as compared to
the control sample, indicating that the combustion process was
carried out to a far greater extent in the exemplary catalyst than
in the control sample. Note also that the unburned hydrocarbons in
the effluent from the control sample was 2.8 volume percent whereas
no unburned hydrocarbons were detected in the effluent from the
exemplary catalyst sample in accordance with the present invention,
despite the fact that the change in temperature was only
335.degree. C. across the exemplary catalyst bed. This modest
temperature increment for a 4% CH.sub.4 in air gas mixture in
conjunction with the substantial elimination of hydrocarbons from
the effluent clearly indicates that a portion of the methane was
thermally combusted downstream of the catalyst bed. It is therefore
seen that the objective of catalytically supported thermal
combustion was attained. Note also that less than 1 ppm nitrogen
oxides was detected in the effluent from the exemplary catalyst
sample despite the fact that all detectable amounts of hydrocarbons
were combusted.
EXAMPLE 5
To illustrate the effectiveness of 2La.sub.2 O.sub.3.sup.. PdO for
methane combustion, its performance was compared to a blank alumina
washcoated honeycomb. Two catalyst bodies measuring 1 inch in
diameter by 8 inches in length were prepared. The first 6 inches of
each body was the same and comprised PdO on alumina catalyst,
coated onto a 64 cell per square inch alumina-silica-magnesia fiber
honeycomb type support. The last 2 inches of a comparative sample
was provided with a plain alumina washcoat, while the last two
inches of an exemplary catalyst composition was provided with about
1.5 g/in.sup.3 of a washcoat containing about 7 weight percent of
the catalytic species 2La.sub.2 O.sub.3.sup.. PdO, balance (93
weight percent) alumina. The conversion of a combustion mixture
comprising 4 volume percent methane in air was measured in a pilot
reactor under three atmospheres pressure and 50 ft per second
linear velocity of gas flow measured at inlet temperature and
pressure. The comparative reactor containing the alumina blank
showed a temperature increase of 200.degree. C. across the reactor
with an inlet gas temperature of 475.degree. C. The exemplary
reactor containing 2La.sub.2 O.sub.3.sup.. PdO produced a
temperature increase of 350.degree. C. across the reactor under the
same inlet gas conditions. The 150.degree. C. greater increase in
temperature across the exemplary reactor as compared to the
comparative reactor is due to improved combustion induced by the
2La.sub.2 O.sub.3.sup.. PdO catalyst. The exemplary catalyst,
comprising only 25% of the length of the bed, induced a 75%
improvement in overall efficiency of combustion as compared to a
reactor in which 25% of the bed was comprised of a blank alumina
catalyst. The exemplary catalyst also supported combustion of
methane at close to 100% efficiency with an effluent containing
less than 5 ppm unburned hydrocarbons and less than 2 ppm nitrogen
oxides, while operating under catalytically-supported thermal
combustion conditions of the type described below with reference to
Pfefferle U.S. Pat. No. 3,928,961. As with the test of Example 4,
the relatively modest temperature increment across the exemplary
reactor and the exceedingly low level of hydrocarbons in the
effluent clearly indicates that desired catalytically supported
thermal combustion was attained, i.e., hydrocarbons were combusted
downstream of the catalyst.
EXAMPLE 6
Preparation of La.sub.2 O.sub.3.sup.. PdO by the Solution Drying
Method
Aqueous solutions of 27.55 g of Pd(NO.sub.3).sub.2 .sup.. 2H.sub.2
O and 22.50 g of La(NO.sub.3).sub.3.sup.. 6H.sub.2 O were slowly
mixed in a beaker with stirring to give a solution containing a 1:1
molar ratio of La to Pd. The beaker was in an oil bath kept at a
temperature between 94.degree. C. and 98.degree. C. The temperature
of the bath was raised to 110.degree. C. and held at that
temperature until the mixed solution evaporated to dryness. The
dried residue was put into a crucible and placed in a muffle oven
set at a temperature of 100.degree. C. which was then raised to
500.degree. C. The residue mixture was kept in the oven at
500.degree. C. for 17.5 hours. After cooling, the mixture was
ground into a fine powder in a mortar using a pestle, to yield a
reaction product containing La.sub.2 O.sub.3.sup.. PdO.
EXAMPLE 7
Testing of the Catalysts Prepared by the Procedure of Example 6
The binary oxide catalyst prepared by the procedure of Example 6
was tested in a laboratory reactor. The catalyst in powder form was
in one case (Part 1 of TABLE III) mixed with alpha alumina
particles to provide a mixture containing 2% by weight of the
reaction product of Example 6 and 98% by weight alumina. In Part 2
of TABLE III, the catalyst powder alone was used. In both cases,
the powder was placed into a quartz tube reactor in which the
catalytic material mixture was supported by a fritted quartz disc.
The inlet combustion mixture test gases used were 1 volume percent
or 0.1 volume percent methane in air. The measure of activity is
the temperature at which 20% of the methane in the inlet combustion
gas had been oxidized. Tests were run on both fresh and aged
catalyst samples and, for comparison, tests were run on beds of
alumina only and with the quartz tube empty. The results attained
are set forth in TABLE III.
TABLE III ______________________________________ Activity Data
Temperature (.degree.C.) at Which 20% Sample Conversion of CH.sub.4
is Attained ______________________________________ 1. Conditions:
1.5 liters per minute of 1% CH.sub.4 in air, 0.06 g catalyst/2.94 g
Al.sub.2 O.sub.3. La.sub.2 O.sub.3.PdO.sup.(a) 574 La.sub.2
O.sub.3.PdO.sup.(b) 605 Al.sub.2 O.sub.3 only 660 Empty tube 721 2.
Conditions: 1 g of La.sub.2 O.sub.3.PdO product, 0.3 liters per
minute of 0.1% CH.sub.4 in air. La.sub.2 O.sub.3.PdO 420 Empty tube
695 ______________________________________ Note: .sup.(a) = Fresh
sample .sup.(b) = After 17 hours of operation at 1100.degree.
C.
TABLE III shows that the activity of the fresh catalyst was still
acceptable after exposure of the catalyst to 1100.degree. C. for 17
hours, even though the test was run at a high space velocity of
1,500,000 hrs.sup.-1.
Generally, the data of TABLE III shows the effectiveness of the
reaction product containing the binary oxide La.sub.2 O.sub.3.sup..
PdO as a catalyst for combustion of dilute mixtures of methane in
air.
EXAMPLE 8
A. In order to evaluate the decomposition and regeneration
temperatures of the catalyst, samples were prepared in accordance
with the technique of Example 6.
B. The resulting samples were subjected to decomposition
temperatures and then treated at lower temperatures in an attempt
to regenerate the catalyst. Thermogravimetric analysis as described
in Example 2 was employed to ascertain decomposition and
regeneration temperatures.
In order to test the regeneration temperature, after the heating
cycle was completed to attain and ascertain the decomposition
temperature, heating was discontinued to allow the heated sample to
cool in air and changes in weight and temperature changes caused by
heats of reaction due to chemical (re-oxidation) and/or phase
changes were monitored during the cooling period.
Referring now to the sole Figure of the drawing there is shown a
graph on which percentage change in weight of the sample subjected
to thermogravimetric analysis is plotted on the abscissa versus the
temperature in degrees Centigrade to which the sample is exposed
plotted on the ordinate. A carefully weighed sample of 20 to 50 mg
by weight of the reaction material obtained in Example 6 was placed
in a quartz pan which was suspended from a weight measuring device
manufactured by Thermal Sciences, Model STA 1500. Air at
approximately 20 cm.sup.3 /min was passed over the sample while a
furnace gradually heated the sample at a rate of about 10.degree.
C./min to attain the temperature shown on the abscissa of the graph
of the sole Figure of the drawing. The weight changes of the sample
occasioned by the heating are plotted as the percentage change in
weight, based on the weight of the unheated sample, on the abscissa
of the chart of the Figure. These show the changes in weight which
result from decomposition of the La.sub.2 O.sub.3.sup.. PdO
compound and consequent loss of oxygen and weight gains occasioned
by re-oxidation of the rare earth oxide La.sub.2 O.sub.3.sup.. PdO
with consequent gain of oxygen and weight. When the desired
temperature is attained the heating was discontinued to allow the
heated sample to cool in air and resultant changes in weight due to
chemical reaction, such as re-oxidation to reconstitute the
La.sub.2 O.sub.3.sup.. PdO compound were monitored during the
cooling period.
With reference now to the Figure, the initial heating of the
reaction product obtained in Example 6 is indicated by the curve
labelled "Cycle 1". The heating period is indicated by the
arrowheads directed rightwardly as viewed in the Figure. While
being heated from about 700.degree. C. to over 900.degree. C., the
weight of the sample remained essentially unchanged but a
precipitous loss in weight occurred starting at about 932.degree.
C., indicating a decomposition of the La.sub.2 O.sub.3.sup.. PdO
compound, and possibly other binary oxides contained in the
reaction mixture. Upon further heating to attainment of a
temperature of about 1080.degree. C., heating was stopped and the
sample was allowed to cool in air, resulting in a further loss in
weight until a temperature of about 790.degree. C. was attained,
wherein a pronounced increase in weight occurred with cooling
between about 790.degree. C. to about 700.degree. C. In a second
heat cycle of the same sample, identified as "Cycle 2" in the
Figure, it is seen that upon conducting a second thermogravimetric
test analysis of the same sample, a significant weight loss is
noted at about 820.degree. C. and stabilizes at about 920.degree.
C. Upon further heating to about 1060.degree. C., the weight
remained fairly stable. Upon being allowed to cool the weight
remained fairly stable until a temperature of about 790.degree. C.
was attained wherein a significant weight increase attributed to
re-oxidation of the palladium oxide species to form catalytically
active La.sub.2 O.sub.3.sup.. PdO is noted; the weight gain
stabilized at about 750.degree. C. It will be noted that in Cycle
2, a much greater degree of recovery is attained than in Cycle 1.
That is, a much higher proportion of the catalytically active
species initially present is recovered by cooling to somewhat below
800.degree. C. The data for continued cycling of the material is
shown in TABLE IV.
TABLE IV ______________________________________
Decomposition.sup.1) Regeneration Cycle Wd % Wd Td Wr % Wr Tr
______________________________________ 1 .70 2.28 955 .16 .52 780 2
.20 .65 790 .17 .55 769 3 .24 .78 870 .18 .58 790 4 .22 .71 790 .17
.55 790 5 .21 .68 790 .16 .52 790
______________________________________ .sup.1) Maximum heating
profile temperature = 1100.degree. C. Wd = Weight loss of sample
during heating cycle, mllligrams. % Wd = Weight loss of sample, as
percent by weight of original weight of sample. Td = Temperature in
.degree.C. at which decomposition takes place. Wr = Weight gain
upon cooling. % Wr = Weight gain of sample, as percent by weight of
decomposed sample. Tr = Temperature in .degree.C. at which weight
gain takes place.
TABLE IV shows that for temperatures less than 1100.degree.
Centigrade, while some La.sub.2 O.sub.3.sup.. PdO is lost on the
initial cycle due to sintering and/or phase changes, some of the
catalytically active material is regenerated at a reasonably high
temperature. By the time the third cycle is attained, a steady
state is achieved and the hysteresis between decomposition and
regeneration of the binary oxide species has been substantially
eliminated. The absence of hysteresis, i.e., the fact that the
compound will commence to regenerate at substantially the same
temperature at which it initially commenced to decompose,
substantially eliminates the hysteresis or "dead zone" in which the
material is a catalytically inactive species. For example, the
palladium oxide catalyst disclosed in U.S. Pat. No. 4,893,465 will,
at atmospheric pressure, decompose at a temperature of about
810.degree. C. and will not regenerate until cooling to a
temperature of about 650.degree. C. is attained, resulting in a 160
degree hysteresis temperature range (between 650.degree. C. and
810.degree. C.) in which the material is substantially
catalytically inactive. The aged La.sub.2 O.sub.3.sup.. PdO binary
oxide, by substantially eliminating this hysteresis or "dead zone"
provides extremely rapid recovery of activity in the case of
accidental over-temperature operation.
Generally, it should be noted that in the practices of the present
invention, neither the binary compound nor any component thereof is
impregnated as a solution of a palladium and/or rare earth metal
salt onto a support material such as activated alumina or the like.
(Activated alumina is comprised mostly of gamma alumina although
other phases are usually present.) It is known in the art of
catalysis to impregnate a high surface material such as particles
of activated alumina with an aqueous solution of a rare earth metal
salt, such as a rare earth metal nitrate, e.g., cerium nitrate.
This impregnation of the alumina particles with the salt solution
is followed by calcination in air, in order to decompose the rare
earth metal nitrate to the oxide, leaving the rare earth metal
oxide dispersed throughout the lattice of the alumina. As is well
known, such impregnation stabilizes the high surface area alumina
against thermal degradation, in which exposure to high temperature
causes a phase change, such as gamma to alpha alumina, resulting in
a collapse of the high surface area structure of the activated
alumina. In contrast, in the practices of the present invention the
binary oxide is mixed with a refractory binder (which may be
alumina) in what may be referred to as "bulk" form. That is, the
binary oxide (as well as the alumina) is in the form of particles
of solid oxide material which are substantially insoluble in
aqueous solution. Thus, solid particles of the binary oxide are
admixed with solid particles of the binder and neither the binary
oxide nor any portion thereof is impregnated into the alumina in
the form of a solubilized precursor of the binary oxide. Indeed,
there is some evidence that at least some of the binary oxides of
the present invention could not exist or be made if it or the
components thereof were to be diffused as a solubilized precursor
into the alumina particles.
Generally, the compounds of this invention may be used to catalyze
combustion of a combustion mixture of oxygen, e.g., air, and a
gaseous carbonaceous fuel, including fuels, such as natural gas,
which contain methane, without significant formation of NOx. Such
combustion of the gaseous carbonaceous fuel may be carried out by
methods known in the prior art as illustrated in, for example, U.S.
Pat. No. 3,928,961 issued Dec. 30, 1975 to William C. Pfefferle.
FIG. 4 of the Pfefferle Patent and the description at column 10,
lines 29-49, disclose a system in which a fuel-air combustion
mixture is introduced into a catalyst zone 34 which may be sized to
provide for combustion of only a minor portion of the fuel therein,
with the major portion of the fuel being combusted by thermal
combustion in a combustion zone 37 located downstream of the
catalyst 34 and of larger volume than the catalyst 34. In this way
combustion is initiated within a catalyst but the majority of the
combustion takes place as thermal combustion in a zone downstream
of the catalyst.
As explained in the aforesaid Pfefferle Patent, conventional, i.e.,
noncatalytic, thermal combustion systems of the type used for
engines and power plants, such as gas turbines, operate at
combustion temperatures which are high enough to form nitrogen
oxides ("NOx") including NO. This is because spark-ignited
flammable mixtures of fuels such as natural gas and methane combust
at temperatures of about 3300.degree. F. (1816.degree. C.) or
higher, which results in the formation of substantial amounts of
NOx from atmospheric nitrogen. Catalytic combustion has the
advantage of occurring at lower temperatures in which the formation
of NOx is avoided or greatly reduced. However, because of
limitations on the rate of mass transfer of the fuel and oxygen to
the catalyst surface, either a prohibitively large surface area of
catalyst must be provided or the mass transfer rate must be
increased to such an extent that an excessively high pressure drop
across the catalyst will be sustained. The Pfefferle Patent
overcomes these difficulties by employing catalytically supported
thermal combustion, based on the finding that if the operating
temperature of the catalyst is increased substantially into the
mass transfer limited region of operation the reaction rate begins
to increase exponentially. Pfefferle theorized that the phenomenon
may be explained by the fact that the catalyst surface and the gas
layer near the catalyst surface are above a temperature at which
thermal combustion occurs at a rate higher than the catalytic rate,
and the temperature of the catalyst surface is above the
instantaneous auto-ignition temperature of the fuel-air admixture.
As a result, the fuel molecules entering this high temperature
layer spontaneously burn without necessity of their being
transported to the catalyst surface. As the combustion process
proceeds, this high temperature gas layer becomes deeper until
substantially the entire gas flow stream of the combustion mixture
is ultimately raised to a temperature at which thermal combustion
reactions occur. Consequently, the thermal combustion takes place
throughout the entire gas stream, not merely adjacent the catalyst
layer. The "instantaneous auto-ignition temperature" as defined by
Pfefferle means that temperature at which the ignition lag of the
fuel-air mixture entering the catalyst is negligible relative to
the residence time in the combustion zone of the mixture undergoing
combustion.
The Patent literature shows numerous further developments and
modifications of the basic system disclosed by Pfefferle, including
William C. Pfefferle U.S. Pat. Nos. 3,940,923, 3,846,979,
3,975,900, and 4,094,142. The disclosure of these Patents and of
U.S. Pat. No. 3,928,961 is hereby incorporated by reference herein.
In such a method, an intimate mixture of the fuel and air is formed
and contacted in a combustion zone with a catalyst composition
comprising the novel compound of this invention. Combustion of at
least a portion of the fuel is thereby attained under essentially
adiabatic conditions at a rate surmounting the mass transfer
limitation to form an effluent of high thermal energy. The
combustion zone may be maintained at a temperature of from about
925.degree. C. to about 1650.degree. C. and the combustion is
generally carried out at a pressure of from 1 to 20 atmospheres.
Configuration of the catalyst to employ the M.sub.2 O.sub.3.sup..
PdO catalysts of the present invention in a zone which during
normal operation is exposed to temperatures which do not exceed
about 950.degree. C. will preclude significant deactivation of the
catalyst during normal operation. Operating upsets which raise the
temperature to levels, e.g., over 950.degree. C., at which the
M.sub.2 O.sub.3.sup.. PdO binary oxide compounds decompose, thereby
inactivating the catalyst, may be rectified by cooling the catalyst
to below about 950.degree. C., e.g., to a temperature within the
range 700.degree. C. to 950.degree. C., e.g., 750.degree. C. to
790.degree. C., thereby re-oxidizing the compound and regenerating
the catalyst. On the other hand, the catalyst reactor may be
configured to include the 2M.sub.2 O.sub.3.sup.. PdO binary oxide
compounds in a zone which during normal operation is exposed to
temperatures which do not exceed the decomposition temperature of
the M.sub.2 O.sub.3.sup.. PdO compound used, e.g., 1200.degree. C.
As shown in Example 3, the onset of decomposition of 2Nd.sub.2
O.sub.3.sup.. PdO is 1230.degree. C. and of 2La.sub.2 O.sub.3.sup..
PdO is 1300.degree. C.
The combustion catalyst of this invention may be used in a
segmented catalyst bed such as described in, for example, U.S. Pat.
No. 4,089,654. Dividing the catalyst configuration into segments is
beneficial not only from an operational standpoint, but also in
terms of monitoring the performance of various sections of the
bed.
Although the invention has been shown and described with respect to
preferred illustrative embodiments, it will be appreciated that
numerous variations thereto will still lie within the scope of the
present invention.
* * * * *